This change replaces std::make_unique with manual construction of
std::unique_ptr to make the tests compatible with C++11
(std::make_unique is a C++14 feature).
libc++ supports std::unique_ptr and std::shared_ptr even in C++03 but
with some limitations: unique_ptr_array.pass.cpp and
shared_ptr_arg.pass.cpp fail to compile in C++03 mode and need to be